What is up with the estimated range based on existing fuel and speed?

[Sgian Dubh]

I will be the first to admit that I have had precious little time to play SHIV and so, with a day off I jumped a the chance today.

So I may be encountering something well known, but it caught me out.

I am on a first patrol (out of Pearl) in mid 1942. It has been an excellent patrol with over 72,000 tons (using manual shooting and lots of deck gun due to misses and dud torpedos - but I have the dud problem in hand now).

I am running 1.4 with TM 1.7.6 (so far).

I had just under a half load of diesel and got a new assigment to partol off Luzon (even though I had just reported no torpedos left and my fuel status). So I decide to head to Midway to re-arm and refuel - but I can't get there. I run out of diesel (running 2/3rds) with over 1300 nm to go.

The range estimator reported (moving at 1/3) with just under a half-load of diesel that my maximum range was under 650nm. So I clicked it again and it came back with a lower number! Each time I clicked it, the number dropped by at least 1nm and then it stabilized about 50 nm less that the original estimate.

So my question is, how much fuel, if you are off the coast of Okinawa, do you need to get back to Midway? It appears to me, based on what I am seeing, that you would need nearly 3/4 of a load of diesel, but it takes nearly half to get from Miday to Okinawa.

I was so irritated that I resorted to something I rarely do with this sim. I rendevous (sp?) with an "oiler" just North and West of Wake Island, so I could make it back to Midway.

I am no novice to sub sims, and I was following all of the usual fuel saving measures. I only had to resort to Full ahead a few times to chase down some contacts during the patrol. I figured I had plenty of fuel left, so I am really puzzled.

Thanks in advance for any advise.